The organization of the DNA sequences of two classes of murine oncogenic viruses within the host cell genome will be studied. In order to define factors stabilizing integrated polyoma genomes within a permissive cell environment, mouse cells transformed by the ts-a mutant of polyoma will be studied and the differences between cells inducible and not inducible for virus production characterized. The arrangement of inserted viral DNA sequences will be deduced from the pattern of viral DNA sequence containing fragments obtained following restriction enzyme digestion and gel electrophoresis. Cell lines infected with or endogenously carrying integrated Moloney murine leukemia virus genomes will be studied in order to determine the number and random or specific nature of the sites for insertion of a C-type RNA viral genome. Cell lines carrying a single inserted Moloney MLV genome which are inducible for virus expression will be developed. Methods for the separation of chromatin into transcriptionally active and inactive fractions will be tested and developed using these cell lines. The localization in fractionated chromatin of single and multiple copies of integrated viral genomes will be studied and related to viral gene expression.